A technology such as Selective Catalyst Reduction (SCR) may be utilized for NOx reduction in engines and to achieve emissions compliance. In one approach, aqueous urea is sprayed into the exhaust gas stream which subsequently reacts with NOx species on the surface of an SCR catalyst, resulting in reduction of engine-out NOx emissions. For improved NOx reduction, the injected urea has to be spread evenly across the surface of the SCR catalyst.
Various approaches may be used to evenly disperse the injected reductant on the surface of the SCR catalyst. One example mixing approach is illustrated by Brück et al. in U.S. Pat. No. 7,380,395. Therein a mixer is included in the exhaust passage between the site of urea injection and the SCR catalyst so that the injected urea is atomized and sufficiently mixed with exhaust gas before it reaches the catalyst substrate. The approach of Brück also includes a filter element with microstructures located upstream of the mixer for generating turbulence to assist in urea mixing. In still other approaches, an atomizer may be included along with a mixer to further improve the atomization and mixing of injected urea with exhaust gas.
However the inventors of the present application have recognized potential issues with such an approach. As one example, the microstructures may not provide sufficient mixing and atomization of the injected urea. Consequently, the system may largely rely on the mixer for providing the desired urea mixing. As another example, the metallic atomizer and mixer can add substantial weight and cost to component manufacture. Additionally, various support structures may be required to hold the mixer and/or atomizer in place in the exhaust passage. The support structures, typically made of metallic materials, may also add further weight and cost to component manufacture. As still another example, the presence of a mixer and/or atomizer, along with the related support structure, can cause a drop in temperature from the time exhaust gas flows out of an upstream catalyst (such as a diesel oxidation catalyst) and into the downstream SCR catalyst. As a result of the drop in exhaust gas temperature, the SCR catalyst may take a longer time to light-off, leading to degraded exhaust emissions.
Accordingly, in one example, some of the above issues may be addressed by an emission control system coupled to an engine exhaust passage comprising a monolithic substrate having a plurality channels of varying cell density, a reductant injector positioned upstream of the substrate wherein injected reductant flows unobstructedly from the injector to the substrate, and an SCR catalytic washcoat positioned on the substrate or downstream of the substrate. It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.